Semiconductor electro-optical detectors are either of the photovoltaic or photoresistive type. Different types of electro-optical detectors are employed for different wavelength regions from infrared through ultraviolet. For example, photovoltaic electro-optical detectors for the infrared wavelength range from approximately 8 to 12 microns and 1 to 5.6 microns are frequently made of mercury cadmium telluride (HgCdTe) and indium antimonide (InSb), respectively. The specific construction of indium antimonide electro-optical detectors is described, for example, in commonly assigned U.S. Pat. Nos. 3,483,096, 3,554,818 and 3,577,175. While the following description is made for InSb electro-optical detectors, the invention in many of its broadest aspects is not limited to this material.
Single element devices, as disclosed in the aforementioned patents, typically include a P-N junction wherein an N-doped bulk substrate carries a P-doped region that is exposed to an optical energy source being detected. Usually, the P-N junction is no greater than about 4 microns from the surface of a P-type region on which the optical energy to be detected is incident. In other words, the P-type region exposed to the optical radiation to be detected has a thickness of no greater than about 4 microns. For certain InSb devices, the P-N junction is closer than 0.5 microns to the surface of the P-type region exposed to the radiation. The P-type region is desirably positioned so that the optical radiation is directly incident thereon to enable photo-generated charge carriers formed in the P-type region to diffuse, somewhat uninterrupted, to the junction. Even in this configuration, a significant amount of optical energy penetrates through the P-type region into the P-N junction where some additional charge carriers are generated and on into the N-type region where still more charge carriers are generated. As long as this absorption in the n-type material does not occur too far away from the junction, the resulting charge carriers also diffuse back to the junction. In addition, this arrangement enables optical energy that is not absorbed in the P-type region to reach the P-N junction directly. Thereby, efficiency in converting optical energy to electric energy is relatively high if the P-doped region of an indium antimonide detector is arranged such that the optical energy is incident on the P-type region.
While these characteristics have long been known, to our knowledge, they have not been achieved when relatively large InSb electro-optical semiconductor detector arrays have been manufactured. In the large InSb array prior art of which we are aware, it has been the practice to illuminate the relatively thick N-type doped bulk substrate semiconductor material, i.e., the "back" face of the array has been illuminated. The thickness of the illuminated N-type doped bulk substrate is typically 10 microns which increases the probability that photogenerated charge carriers will interact with crystal defects or other charge carriers in the N-type bulk substrate. This is particularly true of the shortest wavelength energy to which the optical detector is exposed because the shortest wavelength energy is absorbed closest to the back face, and the resulting photogenerated charge carriers must travel the greatest distance to the P-N junction. In addition, very little, if any, of the optical energy in the 1-4 micron region can propagate unimpeded to the P-N junction through the bulk material.
The construction and manufacturing method of a typical prior art indium antimonide, photovoltaic detector array are illustrated in FIGS. 1 and 2. In this and other prior art arrangements, the optical energy to be detected is first incident on the relatively thick (about 10-20 microns) N-type bulk substrate. Hence, the distance between the P-N junction and the surface on which the optical radiation to be detected is first incident is approximately 10-20 microns. For the shortest wavelengths in the 1-5.6 micron band to be detected, i.e., between 1 and 3 microns, there is a relatively low quantum efficiency because photogenerated charge carriers created in the N-type bulk substrate in response to the incident optical energy do not proceed in an unimpeded manner to the N-type bulk substrate. Instead, the free charge carriers resulting from absorption of optical energy photons frequently interact with the InSb crystal lattice and crystal defects prior to reaching the P-N junction, causing the carriers to lose energy and recombine with other carriers of the opposite type and therefore go undetected. In addition, very little of the shortest wavelength energy is able to reach the junction without being absorbed in the N-type material and create photogenerated carriers therein.
In the prior art arrangement illustrated in FIGS. 1 and 2, an indium antimonide N-type bulk substrate 23, having a thickness of approximately 15 mils with an array of P-type regions 24 formed thereon, is connected to multiplexer substrate 25 by indium columns 26, which can be grown on metal contact pads (not shown), typically of gold, nickel or chromium for the P-type regions or on the multiplexer substrate or on a combination of both. A P-N junction, forming a diode, exists at each location of P-type region 24 on N-type bulk substrate 23. After the detector assembly including N-type bulk substrate 23 and an array of P-type regions 24, formed by gaseous diffusion or ionic bombardment, has been connected to multiplexer substrate 25, the bulk material substrate is mechanically and/or chemically thinned and polished to a thickness of approximately 10 microns, as illustrated in FIG. 2. Multiplexer substrate 25 includes electronic circuitry having switching elements with substantially the same topography as the topography of P-type regions 24. The electronic circuitry in multiplexer substrate 25 selectively reads out the signal from a selected diode of the electro-optical detector array through the indium bump to one or a few common signal leads on the multiplexer chip. This causes readout of the optical energy incident on a surface of N-type bulk substrate 23 corresponding generally with the P-type region 24 connected to the indium column or bump 26 which is selected by the circuitry on multiplexer N-type bulk substrate 25.
To withstand the mechanical forces during and after thinning, an epoxy bonding agent is injected between the array including N-type bulk substrate 23 and P-type regions 24 and multiplexer 25. The bonding agent fills the space between indium columns or bumps 26.
In use, the structure of FIG. 2 is arranged so that the optical energy is initially incident on N-type bulk substrate 23. The optical energy creates free charge carriers, an electron-hole pair for each photon absorbed, in bulk substrate 23. If the minority carrier, i.e., the hole in N-type indium antimonide N-type bulk substrate 23, recombines with a majority carrier, no current results and the optical energy is not detected. If, however, the minority carrier diffuses to and crosses the junction between N-type bulk substrate 23 and a particular P-type region 24, current is produced in the P-type region.
Whether a minority carrier diffuses across the junction is a function of (1) how far away from the junction the electron-hole pair is at the time it is created by the incident optical energy, (2) the diffusion length in bulk material 23, and (3) the density of the bulk material defects which act as recombination centers. These defects can exist before processing is performed. However, other defects are created by several of the many processing steps, e.g., by ion implantation, thinning and/or bump bonding. In general, detector efficiency in converting optical energy to electrical energy decreases as the distance between the junction and the surface on which the optical energy is initially incident increases.
Thinning and polishing operations performed on N-type bulk substrate 23 place severe stresses on the detector and often result in N-type bulk substrate cracking. Polishing compounds and mechanical abrasion used to thin N-type bulk substrate 23 result in microcrystalline damage on the surface of and into bulk material 23 on which the optical energy is initially incident. The microcrystalline damage has severe detrimental effects on the electrical characteristics of the array. The resulting degradation of the bulk material in N-type bulk substrate 23 on which the optical energy is incident produces a high surface recombination rate in the N-type bulk substrate, lowering quantum efficiency dramatically, particularly at the shortest wavelengths which are absorbed close to the surface of N-type bulk substrate 23.
The detector arrays are usually operated at cryogenic temperatures, in the liquid helium or liquid nitrogen range. While lower quantum efficiencies at short wavelengths may not occur in InSb arrays at liquid nitrogen temperatures, the diffusion length (mentioned above) decreases dramatically as temperature is lowered further such that at liquid helium temperature ranges (of interest to astronomers), the performance of the prior art devices is degraded. Also, operation at liquid nitrogen temperatures causes the arrays to undergo severe mechanical strain due to thermal expansion mismatch between the bulk material of N-type bulk substrate 23 and multiplexer substrate 25. The very thin bulk material of N-type bulk substrate 23 cannot always accommodate the induced strain and is subject to breakage or may become deformed to cause bonds between indium columns 26 and N-type bulk substrate 23 or the pads thereon to fail.
An additional disadvantage of the prior art methods is that the thinning process is performed after sawing the detector wafer into individual chips and after bump bonding occurs. Hence, e.g., if there are ten arrays on a wafer, the thinning process is performed ten different times, resulting in an expensive processing technique.
It is, accordingly, an object of the present invention to provide a new and improved method of making electro-optical detector arrays and to a method of making the same.
Another object of the invention is to provide a new and improved method of making electro-optical detector arrays having P-N junctions in very close proximity to a surface on which optical energy is initially incident.
An added object of the invention is to provide a new and improved method of making electro-optical detector for infrared energy over a relatively wide wavelength band, which detector has relatively high quantum efficiency for wavelengths throughout the wide band.
A further object of the invention is to provide a new and improved method of making electro-optical detectors adapted to be used in cryogenic temperature environments, but which has stable mechanical and electrical properties even though the array is subject to temperature cycle extremes.
Still another object of the invention is to provide a new and improved method of making electro-optical detector arrays wherein all detector processing is performed prior to connecting the wafer to external control circuitry, e.g., a multiplexer N-type bulk substrate.
An additional object of the invention is to provide a new and improved method of making indium antimonide detector arrays having relatively high quantum efficiency over the entire spectrum of use for such arrays, wherein the array has stable mechanical and electrical properties, even though it is operated at cryogenic temperatures and is subject to cycling between those temperatures.
Still another object of the present invention is to provide a new and improved method of making indium antimonide detector arrays wherein P-type material in the indium antimonide is positioned so that infrared optical energy to be detected is initially incident on the P-type material, instead of on the N-type material of the detector.